1. Field of the Invention
The present invention pertains to a system for detecting, tracking, displaying, and identifying repetitive movement of the human body, and more particularly, to a method and apparatus for monitoring human performance, including identification of movements, displaying variation in movement patterns, and detecting breathing patterns.
2. Description of the Related Art
Numerous methodologies and related devices exist for tracking movement of the human body, especially in the context of sporting activities, with the goals of improving performance and reducing injuries. One technique uses an accelerometer mounted on the body to detect movement by sensing acceleration and deceleration of the body.
There are two components of acceleration, typically identified as “static acceleration” and “dynamic acceleration.” Static acceleration is prolonged acceleration, usually in one direction, such as the acceleration from gravity; whereas dynamic acceleration is created by rapid variations in velocity, such as caused by vibration and shock. Accelerometers will always detect both static and dynamic acceleration. In the absence of any motion, an accelerometer will always detect a static acceleration, which is the acceleration from gravity. Depending on the conditions under which an accelerometer is used, one of these two components of acceleration will prevail. The static acceleration will be generated from a change in position of the accelerometer with respect to a vertical axis used as a reference. For example, in the case of a swimmer, the motion of the body (rotation of the torso in crawl and backstroke and tilting of the torso in breaststroke and butterfly) will create a static acceleration that is much larger than the dynamic acceleration along the axis of motion resulting from the arm pull. On the other hand, an accelerometer used to measure acceleration and deceleration of a vehicle on a flat, straight road will generally only detect the dynamic acceleration (or deceleration). There will be no static acceleration relative to a vertical axis used as a reference because the position of the vehicle with respect to the vertical axis is unchanged.
One example of an accelerometer used in detecting human movement is described in U.S. Pat. No. 5,685,722 issued to Taba for electronic timing swimmer's goggles. Taba describes a three-axis accelerometer that is supposed to detect absolute variations in dynamic acceleration. The accelerometer is attached to the swimmer's goggles in a position to detect the swimmer's movement along an axis that is parallel to the direction of travel. Using a linear regression analysis method, Taba purports to count the swimmer's laps by determining when the swimmer starts, stops, and performs a turn. One disadvantage of this approach is the limited information it provides. Another disadvantage is poor performance due to the weak signals generated from the accelerometer because monitoring dynamic acceleration along the axis of motion produces very weak signals that tend to be lost or corrupted.
More particularly, Taba asserts that his device can detect the motion of a swimmer along the axis of motion from the dynamic acceleration. This would be true on a subject that moves without creating any static acceleration. An example would be a car or train on a flat, straight path. Because the body of a swimmer in Taba's application is constantly moving at any angle with respect to the vertical axis, a large static acceleration signal is generated that is superimposed on the weak dynamic acceleration signal. To remove this static component, it is necessary to have a fixed reference and have knowledge of the position of the swimmer with respect to the vertical axis at all times in order to subtract the static component from the global signal received by the sensor. Having the sensor attached to the swimmer as Taba teaches does not enable discrimination between the signal amplitude resulting from a change of angle with respect to the vertical axis and signal amplitude resulting from dynamic acceleration. Thus, the three-axis accelerometer as taught by Taba fails to get the swimmer's position from a fixed reference at all times, and when this condition is not met, the motion of the swimmer along the axis of motion cannot be known.
In addition, Taba teaches taking all the points of a received signal over one period and using a linear regression analysis method to characterize these points by two data defining a linear equation (m for slope and b for the linear equation y=m*x+b). Taba purports to repeat this process for a subsequent period and then compare the values of m and b, declaring the periods to be the same when these values are the same. However, Taba fails to teach how periodicity is determined. Without this fundamental teaching, Taba's invention cannot be practiced. In addition, Taba ignores the rupture of periodicity that occurs during starts and turn. Without detecting these ruptures and taking them into account, including extracting them mathematically, which Taba does not disclose, it is not possible to provide accurate and useful data.
Hence, there is a need for a device that produces valid and reliable information regarding continuous repetitive movement, including not just starting, stopping, and turning, but information regarding the type of movement, changes or variation in movement patterns, and other performance parameters, such as breathing patterns.